Alkoxy bridges, oxidation

Upon oxidation, alkoxy bridges seem to be invariably formed and this may be viewed as an internal Lewis base association reaction in response to the increased Lewis acidity of the metal ions which accompanies the changes (MiM)6 (M=M)8 ... 

The alkoxy radicals so formed are the precursors of a, P-unsaturated ketones, aldehydes, carboxylic acids and alcohols. Saturation reactions, Nor-rish type I reactions of ketonic compounds and oxidation of aldehydic species occur under irradiation. This finally leads to the formation of saturated carboxylic acids as the main oxidation products accompanied by esters, y-lactones, peresters, anhydrides, alcohols and ether bridges. 

The reaction of trialkyl alanes with tertiary alcohols is worth mentioning in this connection, as only the first alkyl group is split off. The resulting dialkyl-alkoxy alanes are particularly unreactive because of steric screening of the AIO2AI bridge. They are also astonishingly stable to air oxidation (106, 107). 

The link between the dioxymethylene function and P450 inhibition, the requirement for catalytic activation of the inhibitor, and the fact that the dioxymethylene group is oxidized implicate this function in the inhibitory events. An inhibitory role has been postulated for free radical, carbocation, and carbanion intermediates, but formation of the carbene from the bridge-hydroxylated metabolite or from its radical precursor is most consistent with the results (Figure 7.12). Substituents other than an alkoxy group on the dioxymethylene group suppress complex formation The retention of... 

Sharing alkoxy groups is the easiest way for metal alkoxides to increase the coordination of the metal atom without changing their stoichiometry. In pure alkoxides, coordination expansion currently occurs via the formation of OR bridges. Therefore oligomeric as well as monomeric molecular precursors can be found. Oligomerization depends on physical parameters (concentration and temperature) and chemical factors (solvent and oxidation state of the metal atom or steric hindrance of alkoxide groups) [7]. 

Alkoxy-radicals in ring b, generated from hydroxy-derivatives of B-homo-cholestanes with Pb(OAc)4, attack suitably placed C—H bonds to form trans-annular ether bridges. The main reactions are illustrated in Scheme 16. The 7a/ -alcohol gave only a little of the 7a, 19-oxide, the main product being an unsaturated aldehyde of uncertain structure. All the reactions in Scheme 16 occur through favourable conformations of the seven-membered ring. ... 

Other examples which strongly support the existence of a metal-metal double bond are furnished by the dinuclear alkoxo-bridged dimers (10). Several synthetic procedures have been reported for the preparation of these compounds. They include the oxidation of quadruply bonded tungsten compounds in alcoholic HCl solutions, the oxidation of [W2Cl9] in alcohols, the electrolytic reduction of alcoholic solutions of WCle and the alcoholysis of WCU. Other alkoxy derivatives can be prepared by a simple alcohol exchange reaction."... 

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